1. Field of the Invention
The present invention relates to an optical communication system for performing communication according to data signals by modulating and transmitting optics, and specifically to a phase noise canceling system for reducing phase noises in an optical communication system comprising a phase processing unit.
2. Description of the Related Art
Optical communication systems have made remarkable progress with the development of coherent optical communication systems.
With a coherent optical communication system, the amplitude, frequency, and phase, etc. of a high coherency laser light emitted from a light source such as semiconductor lasers, etc. are modulated in the sending equipment directly by a data signal to be communicated. A resultant modulated light is transmitted through a low-loss, broadband, and nonconductive transmission line such as optical fiber cables. In receiving equipment, a received modulated light is converted by optical heterodyne detection or homodyne detection into an electric signal having a frequency of an intermediate frequency band or a base band frequency band, and a data signal is demodulated from the electric signal.
With the above described coherent optical communication system, a laser light, that is, a carrier light, is directly modulated according to a data signal in a base band. Namely, a data signal is transmitted as a modulation component of a carrier light, and can be received with high sensitivity through optical heterodyne or homodyne detection.
Besides, with a coherent optical communication system, a large light frequency range (light wavelength range) can be used through the optical heterodyne detection, etc. As a result, a multiplexed light frequency (wavelength) transmission system for transmitting a data signal of a plurality of channels can be realized using various carrier lights having different frequencies (wavelengths).
The applicant of the present invention refers to a subcarrier multiplexing (SCM) optical communication system in the patent application of "Tokugan-hei P2-242615" in Japan and in the serial number "07/760,019" of the U.S. Patent Application. In the SCM optical communication system, a data signal of a plurality of channels is frequency- multiplexed at a microwave level in an electrical stage, and a resultant electric signal modulates and transmits a carrier light.
Therefore, a frequency multiplexing operation is not always required in a light stage, but an electrical stage multiplexes a frequency (in sending equipment) or identifies it (in receiving equipment). As a result, a light control unit for performing a high level control in a light stage can be minimized, and a frequency multiplexing optical communication system can be realized at a low cost.
Besides, the above described SCM optical communication system can be combined with a coherent optical communication system using direct modulation of the above described semiconductor laser light, etc. and optical heterodyne detection, etc. With the coherent SCM optical communication system, an optical communication system having a higher channel density and a larger capacity than a conventional coherent optical communication system and a conventional SCM optical communication system can be realized at a low cost.
As described above, the latest optical communication system such as coherent optical communication systems, SCM optical communication systems, and coherent SCM optical communication systems, etc. can be widely used for various applications in a conventional optical communication system, a broadband transmission system for an optical CATV (Cable Television) network and image information, and in any large capacity optical communication network for the future ISDN (Integrated Service Digital Network), etc.
In the above described various optical communication systems, if the frequency or the phase of a carrier light can be modulated by a data signal, random phase noises of the carrier itself or phase noises incorporated in a communication process largely affect the demodulating function of the data signal. Therefore, these phase noises must be appropriately canceled.
Even though the amplitude of a carrier light is modulated by a data signal, the phase noise affects the demodulating function of a data signal if the optical heterodyne detection is performed in the demodulating process and a data signal is transmitted at a low rate.
The phase noise canceling method can be a light frequency canceling method or an intermediate frequency stage (IF stage) canceling method after optical heterodyne (or optical homodyne) detection. In the phase noise canceling method in the light stage, a light circuit is complicated, and the center frequency of a modulated light is exceedingly higher (about 1000 times) than the center frequency in the IF stage after the detection, thereby requiring precise control and hardly guaranteeing its realization. Thus, the phase noise canceling process can be realized in the IF stage much more easily.
FIG. 1 (Prior Art) shows a basic configuration of the conventional coherent optical communication system capable of canceling phase noises.
First, the sending equipment is explained below.
Data signal D is a communication signal in the base band having a predetermined transmission rate B. FIG. 2 (Prior Art) shows its frequency characteristics. Data signal D requires that the main lobe of its frequency component is surely stored. Therefore, in data signal D, harmonic components other than the main lobe are removed by a low-pass filter.
A light modulator 101 modulates using data signal D a carrier light having a light frequency of f.sub.s. The light modulating method is, for example, a phase shift keying (PSK) method. The carrier light is, for example, a laser light from a semiconductor laser (a distributed feedback (DFB) semiconductor laser having a broadband with a narrow width of a spectral line is desirable). The above described light modulating operation can be realized as directly modulating using data signal D the bias voltage of the LiNbO.sub.3 light phase modulator and the bias current or temperature of the semiconductor laser.
A modulated light obtained by the light modulator 101 is transmitted through optical fibers 102.
Next, the receiving equipment is explained below.
A modulated light transmitted through the optical fiber 102 is mixed in a mixing circuit 103 with a local oscillator light beam having a center frequency of f.sub.L from a local oscillator laser divice (Lo-LD) 104 which is a semiconductor laser. The mixed light is optical-heterodyne-detected by being received by a light detector 105 such as photodiodes. In this case, the difference frequency .vertline.f.sub.s -f.sub.L .vertline. between the center light frequency f.sub.s of the modulated light (equal to the center light frequency of the carrier light) and the light frequency f.sub.L of the local oscillator light equals the center frequency f.sub.if of an electric signal in the intermediate frequency stage (IF stage) after the detection. The mixing circuit 103 can be, for example, a half-mirror or fiber-type light coupler.
An intermediate frequency signal output by the light detector 105 is amplified by an amplifier 106, and then applied to a phase noise canceling circuit (PNC) 107 where a phase noise contained in an intermediate frequency signal is canceled.
The intermediate frequency signal in which a phase noise is canceled can be demodulated by a demodulating circuit (DEMOD) 108.
FIG. 3 (Prior Art) shows the configuration of the PNC 107 shown in FIG. 1. In this prior art technology, a carrier component is regenerated from a received intermediate frequency signal. Then, a phase noise can be canceled by comparing the phase between the carrier component and the received intermediate frequency signal.
In an ordinary coherent optical communication system, a data signal in a base band is transmitted as a modulated component of a carrier light as described above. Therefore, in the IF stage after the detection, the band of the frequency f.sub.if of a carrier component and the frequency band of a data signal are included. Accordingly, only the carrier component must be regenerated by an appropriate means.
Thus, in the PNC 107 shown in FIG. 1 and having the configuration shown in FIG. 3, a received intermediate frequency signal is branched to two routes. In the first route, a received intermediate frequency signal is delayed by a delay circuit 301. In the second route, a carrier component is regenerated by a frequency multiplier 302, a band-pass filter 303, and a frequency divider 304. Then, an output signal in each route is mixed by a multiplier 305, thereby generating an demodulated signal with a phase noise canceled.
The principle of the phase noise canceling method based on the configuration shown in FIG. 3 is explained below. The configuration shown in FIG. 3 can be assumed if the light modulator 101 shown in FIG. 1 light-modulates a data signal D, which is a digital signal, by the PSK method. In the following equations, A.sub.1 -A.sub.5 are predetermined constants.
First, the intermediate frequency signal I.sub.1 applied from the amplifier 106 shown in FIG. 1 can be represented by the following equation (1). EQU I.sub.1 =A.sub.1 cos {2.pi.f.sub.if t+.theta.(t)+.theta.(t)}(1)
where .theta.(t) indicates a phase component corresponding to a data signal D light-modulated by the PSK method, and has a phase value of either 0 or .pi.. .phi.(t) indicates a phase noise to be canceled.
The intermediate frequency signal I.sub.1 is branched to two routes.
First, an intermediate frequency signal I.sub.2 delayed by the delay circuit 301 in the first route can be represented by the following equation (2) according to equation (1). ##EQU1## where .DELTA.t.sub.1 indicates a delay time in the delay circuit 301.
An intermediate frequency signal I.sub.3 having the frequency doubled by the frequency multiplier 302 in the second route can be represented by the following equation according to equation (1). EQU I.sub.3 =A.sub.2 cos {2.pi..multidot.2f.sub.if t+2.theta.(t)+2.phi.(t)}
where .theta.(t)=0, .pi.. Therefore, 2.theta.(t)=0, 2.pi.. Accordingly, the above equation can be represented by the following equation (3). EQU I.sub.3 =A.sub.2 cos {2.pi..multidot.2f.sub.if t+2.phi.(t)}(3)
The intermediate frequency signal I.sub.3 is filtered through the band-pass filter (BPF) 303 for passing only the frequency components close to the frequency 2f.sub.if, and then applied to the frequency divider 304. An intermediate frequency signal I.sub.4 output by the frequency divider 304 can be represented by the following equation according to equation (3) above. EQU I.sub.4 =A.sub.3 cos {2.pi..multidot.f.sub.if t+.theta.(t)}
where a time delay arises when an intermediate frequency signal is transmitted through the frequency multiplier 302, the BPF 303, and the frequency divider 304. In consideration of the time delay, an output I.sub.4 of the frequency divider 304 can be represented by the following equation (4) according to the equation above, where .DELTA.t.sub.2 indicates the time delay. EQU I.sub.4 =A.sub.3 cos {2.pi.f.sub.if (t-.DELTA.t.sub.2)+.phi.(t-.DELTA.t.sub.2)} (4)
As described above, an intermediate frequency signal I.sub.4, which is a carrier component, can be regenerated from the second route.
Then, the intermediate frequency signal I.sub.4 is multiplied by the multiplier 305 with the intermediate frequency signal I.sub.2 delayed by the delay circuit 301 in the first route. In the resultant output signals, a component I.sub.5 in the detection band can be represented by the following equation (5) according to the above equations 2 and 4. ##EQU2##
However, .DELTA..phi.(.DELTA.t.sub.1 -.DELTA.t.sub.2) indicates a phase noise contained in the output from the multiplier 305, and can be represented by the following equation (6). EQU .DELTA..phi.(.DELTA.t.sub.1 -.DELTA.t.sub.2)=.phi.(t-.DELTA.t.sub.1)-.phi.(t-.DELTA.t.sub.2)(6)
If the above described phase noise indicates a zero-mean Gaussian white noise, .sigma..sup.2 the variance of the phase deviation can be represented by the following equation (7). EQU .sigma..sup.2 =2.pi..DELTA..nu..sub.if .vertline..DELTA.t.sub.1 -.DELTA.t.sub.2 .vertline. (7)
where .DELTA..nu..sub.if if indicates the line width of a beat spectrum.
According to the above equations (6) and (7), the fluctuation of the phase noise component .DELTA..phi.(.DELTA.t.sub.1 -.DELTA.t.sub.2) can be set to 0 by controlling the delay circuit 301 such that the time delay .DELTA.t.sub.1 in the delay circuit 301 equals the time delay .DELTA.t.sub.2 in the frequency multiplier 302, the BPF 303, and the frequency divider 304. Thus, the output I.sub.5 from the multiplier 305 represented in the above described equation (5) is represented in the following equation (8). EQU I.sub.5 =A.sub.4 cos {.theta.(t-.DELTA.t.sub.1)} (8)
where a phase noise is canceled out. Applying the intermediate frequency signal I.sub.5 to the DEMOD 108 shown in FIG. 1 enables a data signal D to be demodulated with the phase noise canceled.
However, the conventional coherent optical communication system having the configuration shown in FIGS. 1 and 3 requires in the configuration of the PNC 107 the frequency multiplier 302, the BPF 303, and the frequency divider 304, etc. in order to regenerate a carrier component from a received intermediate frequency signal. This offers the problem that the total cost of the system rises considerably.
To guarantee the regeneration of a carrier, the light phase modulation (0, .pi.) must be successfully performed. This is accompanied by the problem of difficulty in techniques.
Next, FIG. 4 (Prior Art) shows the basic configuration of the conventional coherent SCM optical communication system capable of canceling phase noises. First, the sending equipment is explained below.
Each of modulators 401-1-401-N modulates each of the carriers of the different frequencies f.sub.1 -f.sub.N allocated to each channel according to each of the data signals D.sub.1 -D.sub.N. This modulating method can be an amplitude modulation (AM), a frequency modulation (FM), or a phase modulation (PM) if the data signal D is an analog signal, and can be an amplitude shift keying (ASK), a frequency shift keying (FSK), or a phase shift keying (PSK) if the data signal D is a digital signal.
Then, a multiplexer 402 composes each of the channel signals modulated by each of the modulators 401-1-401-N to generate a subcarrier multiplexed signal (an SCM signal). The multiplexer 402 only has to be equipped with a function of adding electric signals, and can be realized from a simple and cheap unit such as a micro-wave coupler, etc.
Each of the channel signals must be frequency-multiplexed in the frequency axis so that signals may not leak to the adjacent channels.
Therefore, in the modulator 401-i (1.ltoreq.i.ltoreq.N), a band-pass filtering process is performed based on each of the frequencies f.sub.i after the modulation such that only the signal component of the main lobe of each data signal D.sub.i can be extracted. That is, if the transmission rate of a data signal D.sub.i is defined as B.sub.i, a band-pass filtering process is performed, where the cut-off frequency is f.sub.i .+-.B.sub.i after the modulation.
Otherwise, in the modulator 401-i, a low-pass filtering process in which a cut-off frequency is defined as B.sub.i can be performed before the modulation such that only the signal component of the main lobe of each data signal D.sub.i in the base band can be extracted.
According to each filtering process by the above described modulators 401-1-401-N, the frequency interval for each channel signal can be approximately double the data transmission rate B.sub.i at minimum for a digital signal.
FIG. 5 (Prior Art) shows the configuration of channels on the frequency axis of an SCM signal obtained by the multiplexer 402.
Next, a light modulator 403 modulates a carrier light having the light frequency of f.sub.s according to an SCM signal output by the multiplexer 402. This light modulating method can be an AM, an FM, or a PM method. As in the case shown in FIG. 1, the carrier light can be a laser light obtained by a semiconductor laser, etc.
The modulated light obtained by the light modulator 403 is transmitted through optical fibers 404.
Next, the receiving equipment is explained below.
A modulated light transmitted through the optical fiber 404 is optical-heterodyne-detected by a mixing circuit 405, a Lo-LD 406, and a light detector 407 in the same configuration as that shown in FIG. 1.
As in the case shown in FIG. 1, the center frequency f.sub.if of the intermediate frequency signal obtained by the light detector 407 is equal to the difference frequency .vertline.f.sub.s -f.sub.L .vertline. between the center frequency f.sub.s of the modulated light from the optical fiber 404 and the light frequency f.sub.L of the local oscillator light. FIG. 6 (Prior Art) shows the configuration of the first-order lower side band IF spectrum having the above described center frequency f.sub.if. As shown in FIG. 6, in the IF stage, each of the subcarrier components having the center frequencies f.sub.if -f.sub.1, f.sub.if -f.sub.2, . . . , f.sub.if -f.sub.N is frequency-multiplied with the unmodulated main carrier component at the frequency f.sub.if.
The above described intermediate frequency signal is applied to the PNC 409 after being amplified by an amplifier 408, and the phase noise contained in the intermediate frequency signal is canceled by a PNC 409.
The intermediate frequency signal is branched to the number of channels after having the phase noise canceled. Each of demodulators 410-1-410-N extracts each of the subcarrier components close to each of the center frequencies of f.sub.if -f.sub.1, f.sub.if -f.sub.2, . . . , f.sub.if -f.sub.N from each of the above described branched center frequency signals, and demodulates each of the data signals D.sub.1 -D.sub.N.
With the above described coherent SCM optical communication system, a frequency-multiplexed signal (SCM signal) is generated by cheap multiplexers such as microwave couplers in the electric stage, and then, the light modulation is performed by the SCM signal. Therefore, only one light modulator is required, and the total cost for the whole system can be considerably reduced. Besides, as described above, the interval of channels can be approximated to double the transmission rate in the sending equipment. Therefore, the receiving equipment can collectively receive all or a plurality of channels using a broadband receiver.
FIG. 7 (Prior Art) shows the configuration of the PNC 409 shown in FIG. 4. As in the coherent optical communication system shown in FIG. 3, this prior art technology extracts a main carrier component from a received intermediate frequency signal. Then, a phase noise can be canceled by comparing the phase of the main carrier component with that of the intermediate frequency signal.
In FIG. 7, a received intermediate frequency signal is branched to two routes. In the first route, the received intermediate frequency signal is delayed by a delay circuit 701. In the second route, the main carrier component is extracted by a band-pass filter 702. Then, a multiplier 703 mixes output signals in both routes, and an intermediate frequency signal in which a phase noise is canceled can be obtained.
The principle of the phase noise canceling method based on the configuration shown in FIG. 7 is explained below. In the following explanation, the light modulator 403 shown in FIG. 4 performs a light modulation in the PM method. In the following equations A.sub.6, A.sub.6 ', A.sub.7, and A.sub.8 indicate predetermined constants.
First, an intermediate frequency signal I.sub.6 applied by the amplifier 408 shown in FIG. 4 can be represented by the following equation (9). ##EQU3##
As described above, D.sub.i (t) (1.ltoreq.i.ltoreq.N) indicates a data signal of each channel, f.sub.if indicates a main carrier frequency in the IF stage, f.sub.i (1.ltoreq.i.ltoreq.N) indicates a carrier frequency in the electric stage corresponding to each channel (FIG. 5), and (f.sub.if -f.sub.i) indicates a subcarrier frequency in the IF stage. m indicates a modulation index or an PM modulation index, .phi.(t) indicates a phase noise to be canceled, and J.sub.n (n=0,1) indicates the n-th order Bessel function of the first kind.
The intermediate frequency signal I.sub.6 is branched to two routes.
First, an intermediate frequency signal I.sub.7 delayed by the delay circuit 701 in the first route can be represented by the following equation (10) according to equation (9). ##EQU4## where .DELTA.t.sub.1 indicates delay time in the delay circuit 701 as in the case shown in FIG. 3.
The BPF 702 in the second route only passes the frequency component close to the main carrier frequency f.sub.if. An intermediate frequency signal I.sub.8 output from the second route can be represented by the following equation according to equation (9). EQU I.sub.8 =A.sub.7 'J.sub.0 (m) cos {2.pi..multidot.f.sub.if t=.phi.(t)}
A time delay arises when the intermediate frequency signal is transmitted through the BPF 702. An output I.sub.8 from the BPF 702 can be represented by the following equation (11), where the time delay is defined as .DELTA.t.sub.2, according to the equation above. EQU I.sub.8 =A.sub.7 J.sub.0 (m) cos {2.pi..multidot.f.sub.if (t-.DELTA.t.sub.2)+.phi.(t-.DELTA.t.sub.2)} (11)
As described above, the intermediate frequency signal I.sub.8, which is a main carrier component, can be regenerated from the second route.
Then, the intermediate frequency signal I.sub.8 is multiplied by the multiplier 703 by the intermediate frequency signal I.sub.7 delayed by the above described delay circuit 701 in the first route. In the resultant output signals, a component I.sub.9 in the detection band can be represented by the following equation (12) according to equations (10) and (11). ##EQU5## where .DELTA..phi.(.DELTA.t.sub.1 -.DELTA.t.sub.2) and .sigma..sup.2 can be obtained in the same manner as in equations (6) and (7) shown in FIG. 3. Therefore, as in the case shown in FIG. 3, the phase noise component .DELTA..phi.(.DELTA.t.sub.1 -.DELTA.t.sub.2) can be set to 0 by controlling the delay circuit 702 such that the delay time .DELTA..sub.2 in the delay circuit 701 equals the time delay .DELTA.t.sub.2 in the BPF 702. Accordingly, the output I.sub.9 from the multiplier 703 shown in equation (12) above can be represented by the following equation (13). EQU I.sub.9 =A.sub.8 .SIGMA. cos {2.pi.f.sub.if (t-.DELTA.t.sub.1)-D.sub.i (t-.DELTA.t.sub.1)} (13)
Applying the intermediate frequency signal I.sub.9 to each of the DEMODs 410-1-410-N shown in FIG. 1 enables a data signal D.sub.i to be demodulated with the phase noise canceled.
However, with the conventional coherent SCM optical communication system having the configuration shown in FIGS. 4 and 7, the PNC 409 extracts the unmodulated main carrier component of the frequency f.sub.if as shown in equations (9) through (13) above. At this time, each of the subcarrier components having the center frequencies f.sub.if -f.sub.1, f.sub.if -f.sub.2, . . . , f.sub.if -f.sub.N must be processed correspondingly.
The power of the unmodulated main carrier component at the frequency f.sub.if shown in FIG. 6 is larger than the power of each of the subcarrier components having the center frequencies f.sub.if -f.sub.1, f.sub.if -f.sub.2, . . . , f.sub.if -f.sub.N. That is, the former has a larger value of (J.sub.0 (m)/J.sub.1 (m)).sup.2. Therefore, the frequency interval between the frequency f.sub.if of the main carrier component and the center frequency f.sub.if -f.sub.1 of the first subcarrier component must be sufficiently wide (about several giga Hz). If the frequency interval is not sufficient, a number of high-degree modulation components enter the reception band, thereby badly reducing the reception sensitivity.
For the above described reason, the optical heterodyne detection circuit comprising the mixing circuit 405, the Lo-LD 406, the light receiver 407, and the amplifier 408 requires a broadband circuit for simultaneously receiving the main carrier components and the subcarrier components. Thus, it has a problem that the total cost for the whole system soars.